The present invention relates to printed circuit wiring boards, and more particularly, to multi-layer printed circuit wiring boards (sometimes referred to herein as multi-layer printed wiring boards, PWB) for microwave circuits employing microstrip line and/or stripline techniques, and a method for making same.
Circuity designed for high frequency application, including radars, transponders, . . . in the gigahertz range, cannot be designed utilizing lumped elements such as resistors and capacitors. Various design techniques are utilized including techniques well known in the art as microstrip and stripline. Microstrip consists of a wire above a ground plane, being analogous to a two-wire line in which one of the wires is represented by an image in the ground plane of the wire that is physically present. The metallic strip conductor is bonded to a dielectric sheet, and a metallic ground plane (or plate) is bonded to the other side of the dielectric sheet. [Strip transmission lines differ from microstrip in that a second ground plane is placed above the strip conductor (or conductor strip).] Conductor pads, having a variety of geometric patterns, are also bonded to the dielectric and exhibit resistive, capacitive, and inductive properties at the frequency range of interest thereby forming the high frequency circuit. The dielectric sheet utilized for these high frequency circuit include fiberglass of various types having a variety of parameters. Fiberglass has a dielectric constant which is about 4, and has high power loss (an order of magnitude as that of PTFE) and the size of the elements on the fiberglass sheet would be substantially bigger.
Polytetrafluoroethylene (PTFE), more commonly known as "TEFLON", a trademark of the Dupont Co., has also been used for the dielectric sheet of circuits of 1 gigahertz (GHZ) or higher. Generally, PTFE has low power loss at the high frequencies as compared to fiberglass. Further, the dielectric constant of PTFE can range in value from about 2 to 11 and is very constant across the entire printed wiring board and constant from lot-to-lot.
However, there are some drawbacks with the use of PTFE dielectric sheets for PWBs. These boards are very flexible, unless brass backed, which adds weight and cost. The flexibility of these PWBs cause solder joint failures due to handling. Signal routing, which includes bias lines, modulation, . . . , is accomplished utilizing "feed-thru" capacitors which are expensive, costly to install, and very unreliable due to the internal multi-layer ceramics make-up of such capacitors. Multi-layered PWBs for high frequency applications currently exist utilizing multi-layers of PTFE in an attempt to reduce board flexibility and signal routing; however, these boards are very costly due to the high cost of PTFE.
Thus, it is desired to provide a new and unique printed wire board which is reliable, eliminates the need for utilizing feed-thru capacitors, is substantial rigid thereby eliminating solder joint failures due to flexure, has a high dielectric constant permitting the size of the PWB to remain relatively small and to have the low power loss, and permits signal lines and transmission lines carrying bias, modulation, . . . to be routed internally as a stripline thereby eliminating the need for coaxial cables and wires including the hand soldering of the connections made by the coax cables and wires. The new and unique board of the present invention solves the above-mentioned problems by providing a "hybrid" board; namely, a board constructed of two dissimilar materials --PTFE and fiberglass. The board flexibility is greatly reduced increasing board reliability, and the cost of the board is substantially reduced. Further, both microstrip and stripline techniques are used on the "hybrid" board.